The overall goal of this application is to investigate the role of Mdm2 and its homolog Mdm4 in maladaptive cardiac hypertrophy and the associated apoptosis. Mdm2 is an E3 ubiquitin ligase, which promotes the proteasome-mediated degradation of the tumor suppressor transcription factor p53. Mdm2 not only promotes p53 ubiquitination, but also binds directly to p53 and blocks its transcriptional activity. In contrast to this dual effect of Mdm2, Mdm4 inhibits p53 activity only through direct protein-protein interactions and not by promoting ubiquitination. These effects of Mdm2 and Mdm4 on p53 have already been confirmed in vivo, since heart-specific deletion of Mdm2 is embryonic lethal, whereas heart-specific deletion of Mdm4 causes dilated cardiomyopathy, and both are rescued by p53 deletion. Based on these data, our working hypothesis is that Mdm2 and Mdm4 are the upstream regulators of p53 in dilated cardiomyopathy. In line with this, our recent studies revealed that the proapoptotic Bcl-2 family member Puma, which is a downstream target of p53, is a critical regulator of cardiomyocyte apoptosis and may serve as an effector of p53 in dilated cardiomyopathy. We also observed in preliminary studies that the transcription factor NFAT involved in pathological hypertrophy is a new substrate of Mdm2. To investigate these hypotheses, we study already available knockout mouse models of Mdm4 and Puma, induce pathological hypertrophy by transverse aortic constriction and assess cardiac function by echocardiography. Stemming from our published data and preliminary experiments, our studies will proceed according to the following Specific Aims. 1) In Aim 1 we will investigate the proteasomal regulation of Mdm2 and NFAT degradation, since Mdm2 may regulate cardiac hypertrophy through NFATdegradation. 2) In Aim 2 we will determine whether Puma deletion rescues apoptosis in p53-dependent dilated cardiomyopathy models, such as Mdm4 deletion and transverse aortic constriction (TAC). This proposal is significant in multiple ways. First, the results we obtain will provide important basic scientific information about the function of the Mdm2-p53 and Mdm4-p53 pathways in cardiac myocytes. This will ultimately aid in the design and development of anti-hypertrophic and/or anti-apoptotic drugs or other treatment strategies suitable in cardiac disease therapy. Furthermore, the Mdm2/p53 and the Mdm4/p53 interface are also targets of recent drug discovery efforts in cancer therapy. Therefore, functional analysis of the Mdm2/Mdm4 system in the heart would contribute to the development of efficient and safe drugs or treatment protocols that have minimal cardiotoxic side effects.